The invention provides a process for determining surface contamination of polycrystalline silicon.
On the industrial scale, crude silicon is obtained by the reduction of silicon dioxide with carbon in a light arc furnace at temperatures of about 2000° C.
This affords “metallurgical grade” silicon (Simg) having a purity of about 98-99%.
For applications in photovoltaics and in microelectronics, the metallurgical grade silicon has to be purified.
For this purpose, it is reacted, for example, with gaseous hydrogen chloride at 300-350° C. in a fluidized bed reactor to give a silicon-containing gas, for example trichlorosilane. This is followed by distillation steps in order to purify the silicon-containing gas.
This high-purity silicon-containing gas then serves as a starting material for the production of high-purity polycrystalline silicon.
The polycrystalline silicon, often also called polysilicon for short, is typically produced by means of the Siemens process. This involves heating thin filament rods of silicon by direct passage of current in a bell-shaped reactor (“Siemens reactor”), with introduction of a reaction gas comprising a silicon-containing component and hydrogen.
The silicon-containing component of the reaction gas is generally monosilane or a halosilane of the general composition SiHnX4-n (n=0, 1, 2, 3; X=Cl, Br, I). It is preferably a chlorosilane, more preferably trichlorosilane. Predominantly SiH4 or SiHCl3 (trichlorosilane, TCS) is used in a mixture with hydrogen.
In the Siemens process, the filament rods are typically inserted perpendicularly into electrodes present at the reactor base, through which they are connected to the power supply. Every two filament rods are coupled via a horizontal bridge (likewise composed of silicon) and form a support body for the silicon deposition. The bridge coupling produces the typical U shape of the support bodies, which are also called thin rods.
High-purity polysilicon is deposited on the heated rods and the bridge, as a result of which the rod diameter grows with time (CVD=Chemical Vapor Deposition/gas phase deposition).
After the deposition has ended, these polysilicon rods are typically processed further by means of mechanical processing to give fragments of different size classes, optionally subjected to a wet-chemical purification and finally packed.
The polysilicon can, however, also be processed further in the form of rods or rod pieces. This is especially true for the use of the polysilicon in an FZ process.
In addition, another known method is to expose small silicon particles directly to such a reaction gas in a fluidized bed reactor. The polycrystalline silicon produced is in the form of granules (granular poly).
Polycrystalline silicon (polysilicon for short) serves as a starting material in the production of monocrystalline silicon by means of crucible pulling (Czochralski or CZ process) or by means of zone melting (float zone or FZ process). This monocrystalline silicon is divided into wafers and, after a multitude of mechanical, chemical and chemomechanical processing operations, used in the semiconductor industry for manufacture of electronic components (chips).
More particularly, however, polycrystalline silicon is increasingly being required for production of mono- or multicrystalline silicon by means of pulling or casting processes, this mono- or multicrystalline silicon serving for production of solar cells for photovoltaics.
Since the quality demands on polysilicon are becoming ever higher, quality control over the entire process chain is indispensible. The material is analyzed, for example, with regard to contaminations with metals or dopants. Contamination in bulk should be distinguished from contamination at the surface of the polysilicon fragments or rod pieces.
It is customary to convert the polysilicon produced to monocrystalline material for the purposes of quality control. In this case, the monocrystalline material is analyzed. Here too, metal contaminations, which are assessed particularly critically in the customer processes in the semiconductor industry, are of particular significance. The silicon is, however, also analyzed with regard to carbon and dopants such as aluminum, boron, phosphorus and arsenic.
Dopants (B, P, As, Al) are analyzed by means of photoluminescence to SEMI MF 1398 on an FZ single crystal produced from the polycrystalline material (SEMI MF 1723).
As an alternative, low-temperature FTIR (Fourier Transform IR spectroscopy) is used (SEMI MF 1630).
The fundamentals of the FZ process are described, for example, in DE-3007377 A.
In the FZ process, a polycrystalline stock rod is gradually melted with the aid of a high-frequency coil, and the molten material is converted to a single crystal by seeding with a monocrystalline seed crystal and subsequent recrystallization. In the course of recrystallization, the diameter of the single crystal forming is first increased in a cone shape (cone formation) until a desired final diameter has been attained (rod formation). In the cone formation phase, the single crystal is also mechanically supported in order to take the load off the thin seed crystal.
A wafer is cut off the monocrystalline rod produced by means of FZ from a polycrystalline silicon rod (SEMI MF 1723). A small wafer is cut out of the pulled mono-crystalline rod, etched with HF/HNO3, rinsed with 18 MOHm water and dried. The photoluminescence measurements are conducted on this wafer.
FTIR (SEMI MF 1188, SEMI MF 1391) enables the determination of carbon and oxygen concentrations.
This involves cutting a small wafer out of a poly-crystalline rod. The wafer is polished. Subsequently, the carbon content is determined by means of FTIR spectroscopy.
Both processes (photoluminescence and FTIR) serve exclusively for determination of contaminants in bulk.
Contaminants at the surface can be determined only indirectly.
DE 41 37 521 A1 describes a process for analyzing the concentration of contaminants in silicon particles, which comprises adding particulate silicon to a silicon vessel, processing the particulate silicon and the silicon vessel to give monocrystalline silicon in a float zone, and determining the concentration of contaminants present in the monocrystalline silicon. The concentrations of boron, phosphorus, aluminum and carbon in the silicon vessel used were determined and give a reproducible background value.
The values for boron, phosphorus and carbon found by means of FTIR by the float zone process were corrected by the proportion which originated from the silicon vessel.
In this application, it is also shown that the fragmentation of a polycrystalline silicon rod leads to contamination of the silicon. This is possible by virtue of silicon fragments being introduced into the silicon vessel, subjected to the float zone process and then analyzed for contaminants by means of FTIR. Since the contamination of the base material prior to fragmentation is known, the additional contamination resulting from the fragmentation can be concluded.
DE 43 30 598 A1 likewise discloses a process which enables the contamination of silicon resulting from comminution processes to be concluded. A silicon block was broken into lumps. The silicon lump was subsequently subjected to a zone melting process and converted to a single crystal. A wafer was sawn out of the single crystal and analyzed for boron and phosphorus by means of photoluminescence. Compared to the average boron and phosphorus contents of the silicon block used, an increase in the boron and phosphorus concentrations is found, which is attributable to factors including the comminution process.
The processes described, however, do not take into account the fact that the environment in which not only the comminution process but also other process steps such as storage, transport, cleaning and packaging take place also has an influence on the contamination of the silicon, especially on the surface contamination thereof.
A purely analytical process for test purposes is inadequate in this regard.
The problems described gave rise to the objective of the invention.